Classification of receptor subtypes is crucial for selective therapy, for understanding the mode of drug action and for defining tissue physiology. Understanding modulation of receptor function, i.e. the signal transduction mechanism, functional interaction among multiple receptors on a given cell and desensitization permits therapeutically relevant manipulation of drug action in vivo. The long term goals of this proposal are to delineate the underlying mechanisms of signal transduction triggered by three 5-HT receptor subtypes in mammalian tissues: the 5-HT1A and 5-HT2 receptors along with a putative receptor subtype, the 5-HT4 and to study the modulation of agonist action at these receptors, both receptor modulation as well as modulation of effector systems. Toward these goals 1) the 5- HT4 receptor will be classified in three mammalian preparations and its effector(s) will be identified; 2) two endogenous processes that participate in regulation of 5-HT2 receptor function (functional antagonism and desensitization) will be characterized; and 3) the initial steps in signal transduction of the 5-HT1A receptor will be elucidated. The 5-HT4 receptor which is negatively linked to adenylate cyclase activity in brain, will b classified by 5-HT agonists and antagonists. Ex vivo experiments with pertussis toxin and receptor-stimulated binding of GTP-gamma-S will test whether G-proteins are linked to the 4-HT4 receptor. The coexistence of the 5-HT4 and the 5-HT2 receptors in the human saphenous vein, both of which mediate smooth muscle contraction, provides a unique opportunity to test whether these distinct receptors share the same intracellular post- receptor mechanism by assaying their sensitivity to Ca++ channel ligands. The mechanisms of 5-HT2 receptor regulation by desensitization and by functional antagonism will be pursued by novel kinetic methods which enable us to quantitate the dynamics of multiple state conversion. Structural elements require for recognition (binding) and for activation of the 5-HT1A receptors will be evaluated by testing analogs of the novel anxiolytic drug, buspirone. These concerted studies of three function 5-HT receptors in mammalian tissues are expected to provide an insight into the pharmacology and physiology of 5-HT receptors in mammalian tissues are expected to provide an insight int the pharmacology and physiology of 5-HT receptors and their relationship to human in health and disease states.